Calibrating TDS meter?

Okay, I'm super confused now. I bought an inline TDS meter for my RO filter and a new handheld meter (Aquapro) to replace my TDS-EZ after it began to show 1-2ppm without being submerged in anything (I think the whole thing accidentally got dunked under water, which is why I opted for the water resistant Aquapro to replace it).

I also bought the 342ppm calibration fluid. My old TDS-EZ reads it as 407ppm. My new "factory calibrated" Aquapro reads it at 416ppm. The inline meter (also "factory calibrated") reads this fluid at 558ppm!

Should I recalibrate any of these? I thought if nothing else the calibration fluid would assure me they were accurate, but now I'm less convinced.. all of them say they shouldn't need calibration out of the box. The TDS-EZ is only a few months old (purchased in October).

Your TDSEZ isn't temperature compensated, your AP1 is, and your inline is compensated to the air temperature or not at all - so for that reason alone you'll likely see a little differnece among the three.

The inlines are calibrated in a moving medium - I doubt you're doing that.

The instructions for the inline meter say that if a flowing solution isn't possible for the calibration, that it should be calibrated to 3% above the calibration solution, which is 352ppm, which is still far from what it reads. The probes are aligned correctly (I did read the instructions).

The TDS-EZ reads fairly close to the AP1 in everything else apart from the fact that it reads 1-2ppm in air... I'm still not sure why it is doing that.

The AP1 was reading the 416ppm of the calibration solution at 76.7˚ - I figured that was as close to 77˚[censored]as I needed to get it... but it's still a larger difference than I would have expected.

I'm just not sure if I should trust the calibration solution or not.. and if that means my meters all need calibrating (despite being "factory calibrated").

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